1. Field of the Invention
The present invention relates to an improvement of a semiconductor laser device and a method for manufacturing the same and, more particularly, to a semiconductor laser device using a III-V group semiconductor material containing aluminum as a constituent element and a method for manufacturing the same.
2. Description of the Prior Art
Recently, a compound semiconductor laser has been developed wherein a III-V group semiconductor material is used to allow continuous oscillation at room temperature. In the semiconductor laser of this type, it is very important, for widening the range of applications, to control a transverse mode by a built-in waveguide structure and to obtain a laser beam of stable basic transverse mode oscillation which is free from astigmatism. A rib-waveguide laser shown in FIG. 1 is known as a typical example of a semiconductor laser having such a built-in waveguide structure. Referring to FIG. 1, reference numeral 1 denotes a semiconductor substrate; 2, a first clad layer; 3, an active layer; 4, a second clad layer; 5, an ohmic contact layer; 6, a current limiting insulating layer; 7 and 8, electrodes; and 9, a diffusion layer. The laser has a waveguide which is obtained by forming a projection (rib) 3a on the upper surface of the active layer 3 so as to change a refractive index. By changing a width W, a height H and a step difference .DELTA.H and compositions of the respective layers, a desired transverse mode can be obtained. However, a difference .DELTA.n between the refractive indices of the waveguide rib portion and the remaining portion is greatly increased in accordance with an increase in the step difference .DELTA.H. Therefore, in order to obtain the basic transverse mode, a strict manufacturing condition of, for example, .DELTA.H.ltoreq.500 .ANG. is required (T. P. Lee et al: IEEE J. Quantum Electron. QE-11, 432 (1975)). For this reason, it is difficult to manufacture the rib-waveguide laser of this type.
In order to solve the above problem, a semiconductor laser shown in FIG. 2 has recently been proposed wherein an optical waveguide layer 10 which has a refractive index smaller than that of an active layer 3 and greater than that of a clad layer 4 is formed in contact with the active layer 3, and a rib 10a is formed on the optical waveguide layer 10. Referring to FIG. 2, the remaining reference numerals are the same as those in FIG. 1. More particularly, reference numeral 1 denotes a semiconductor substrate; 2, a first clad layer; 5, an ohmic contact layer; 6, an insulating layer; 7 and 8, electrodes, respectively; and 9, a diffusion layer.
In this laser, the manufacturing condition becomes less strict such that .DELTA.H .ltoreq.0.2 .mu.m, and the manufacturing method is simplified. This laser has advantages in that the basic transverse mode can be easily obtained, and that the beam irradiation angle becomes small to obtain a high optical output.
However, in the laser of the type described above, when a material such as Al.sub.x Ga.sub.1-x, Al.sub.x Ga.sub.1-x AsP and In.sub.y (Al.sub.x Ga.sub.1-x).sub.1-y P each containing aluminum as one of the constituent elements is used to form the optical waveguide layer 10, the structure shown in FIG. 2 cannot be obtained in good reproducibility. In order to obtain the structure shown in FIG. 2, crystal growth is performed up to the optical waveguide layer 10, and the rib 10a is formed on the optical waveguide layer 10 by etching or the like. Thereafter, the second clad layer 4 and the ohmic contact layer 5 are epitaxially grown, and the current limiting insulating layer 6 and the diffusion layer 9 are formed in the name ordered. Therefore, accurate mask alignment must be performed to form a resist film on a portion of the ohmic contact layer 5 which corresponds to the rib 10a.
In this case, since the rib 10a is buried in masking, the position of the rib 10a cannot be directly identified. Therefore, edges 5a of the projection of the ohmic contact layer 5 which is formed in correspondence with the shape of the rib can be used as a reference for masking.
However, when the second clad layer 4 and the ohmic contact layer 5 are formed by conventional liquid phase epitaxy (LPE), a protrusion having wave-like zigzag side edges of these layers 4 and 5 would be formed. This is considered to be due to local non-uniformities in crystal growth rate of these layers when LPE method is employed. Once an optical waveguide layer 10 containing aluminum is exposed to air atmosphere during the aforementioned etching step, an oxide layer is readily formed on the surface of the optical waveguide layer. As a result, the second clad layer 4 cannot be appreciably grown on such an oxidized surface. Even if the second clad layer 4 is grown to some degree, the non-uniformity of the second clad layer 4 becomes more conspicuous as compared with the case where aluminum is not included in the optical waveguide layer 10. Hence, masking for forming the current limiting region tends to become inaccurate, thereby deteriorating the performance and yield of the laser. In this sense, it was considered that the laser having the structure shown in FIG. 2 can be obtained when an InGaAsP semiconductor material which does not contain aluminum is used and the above laser cannot be obtained when a semiconductor material contains aluminum.